This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-20191 filed Jan. 29, 2001.
1. Field of the Invention
The present invention relates to a navigation system, a GPS terminal used in this system and a navigation method, which uses supplementary data transmitted in a broadcast radio wave signal.
2. Background of the Invention
The GPS (Global Positioning System) is now used for measuring, on the ground, the present position of a mobile object such as an automobile, aircraft or vessel with higher accuracy as data such as latitude, longitude and altitude from information pieces included in radio wave signals transmitted from a plurality of GPS satellites revolving on the satellite orbits in the altitude of 20,000 km or higher from the earth by receiving such radio wave signals, for example, by a mobile object moving on the ground.
The information pieces included in the radio wave signals transmitted from the GPS satellites further include navigation messages consisting of the C/A (Coarse/Acquisition) code, Almanac data and Ephemeris data or the like. The Almanac data is the data in relation to rough orbit information of all GPS satellites, while the Ephemeris data is detail data for detecting the position on the orbit of the GPS satellite which has transmitted the radio wave signal.
In general, a GPS terminal receives first all Almanac data transmitted from the GPS satellites before starting execution of navigation. When it is determined appropriate to seize the satellite at the present position from such Almanac data, about four GPS satellites, for example, are selected to obtain the necessary navigation messages by seizing these four GPS satellites.
However, setup of navigation function is delayed because the time of about 12 to 13 minutes are required to receive all Almanac data. Therefore, following services are provided in a part of the system. Namely, a base station installed on the ground always receives the GPS signals transmitted from the GPS satellites, extracts the Almanac data from the navigation messages analyzed, and stores such data in the servers prepared for users.
Here, a GPS terminal user starts, upon acquisition of the Almanac data by making access to the server via a mobile telephone set before driving an apparatus to execute the navigation function, seizure of appropriate GPS satellites based on the acquired Almanac data. With provision of such services, the GPS terminal user can acquire the Almanac data within a short period of time and also can quickly set up the navigation function.
However, in the above system, various procedures are required for users to acquire the Almanac data. It is very troublesome to actively make access to servers. Moreover, an interface is required in the GPS terminal side in order to set up the connection to a mobile telephone system apparatus or the relevant apparatus or the like. Accordingly, the apparatus is complicated.
It is therefore an object of the present invention to provide a navigation system which can quickly set up navigation function of a GPS terminal in a simple structure, and also provide a GPS terminal to be used in the same system.
According to the present invention, a base station receives, on the ground, GPS signals transmitted from GPS satellites. A one-way transmitting station transmits, to the predetermined area, supplementary information including Almanac data in navigation messages of GPS signals received by the base station using radio wave signals. A GPS terminal starts, upon reception of the supplementary information transmitted from the transmitting station, reception of the GPS signals transmitted from the GPS satellites.
When the GPS terminal side is provided with a function to receive the radio wave signals transmitted from the transmitting station, the Almanac data which is already acquired with the base station can be obtained as the supplementary information. Therefore, the Almanac data can be obtained within a short period of time with the GPS terminal, and thereby the navigation function can be set up quickly.